1. Field of the Invention
The present invention relates to surface acoustic wave filters for use as band-pass filters in, for example, mobile communications systems, and more particularly, to a ladder-type surface acoustic wave filter in which a plurality of one-port surface acoustic wave resonators are connected to define a ladder-type circuit.
2. Description of the Related Art
Conventionally, various surface acoustic wave filters have been proposed as band-pass filters. For example, Japanese Patent No. 56-19765 and Japanese Unexamined Patent Application Publication No. 9-246911 disclose surface acoustic wave filters having ladder-type circuit configurations.
FIG. 16 illustrates the basic circuit configuration of a ladder-type surface acoustic wave filter. As shown in FIG. 16, in the surface acoustic wave filter, a signal line extending between an input terminal 101 and an output terminal (not shown) defines a series arm, and at least one series arm resonator 102 is connected to the series arm.
Furthermore, between the series arm and the ground potential, a signal line defining a parallel arm is connected, and a parallel arm resonator 103 is connected to the parallel arm. The series arm resonator 102 and the parallel arm resonator 103 are respectively defined by a one-port surface acoustic wave resonator.
Although FIG. 16 shows a single-stage circuit configuration, however, multiple stages are usually provided in the ladder-type circuit.
The one-port surface acoustic wave resonator typically includes an interdigital transducer (IDT) disposed at a central portion along the direction of surface acoustic wave propagation, and reflectors disposed on both sides of the IDT along the direction of surface acoustic wave propagation.
FIG. 17 is an illustration showing an example of filter characteristics of a surface acoustic wave filter having the conventional ladder-type circuit configuration.
Japanese Patent No. 56-19765 discloses a surface acoustic wave filter having a ladder-type circuit configuration as described above. The surface acoustic wave filter provides a low insertion loss and a wide pass band, and is thus suitably used as a band-pass filter for cellular phones, and other suitable devices.
In the surface acoustic wave filter, the IDT is defined by a pair of comb electrodes arranged such that electrode fingers thereof are interdigitated and such that the overlap lengths of the electrode fingers of the IDT are constant. That is, an IDT in a one-port surface acoustic wave resonator usually is a normal-type IDT.
In a surface acoustic wave resonator having a normal-type IDT, because the overlap lengths of electrode fingers are constant as described above, higher-mode surface acoustic waves, as well as fundamental-mode surface acoustic waves intended to be used, are excited. Generally, the phase velocity of higher-mode surface acoustic waves is larger than the phase velocity of fundamental-mode surface acoustic waves, and therefore, resonance caused by the higher-mode surface acoustic waves appears as sub-resonance on the higher-frequency side as compared with resonance caused by the fundamental-mode surface acoustic waves.
Thus, regarding the filter characteristics of the resonator-based surface acoustic wave filter having the ladder-type circuit configuration, the higher-mode resonance appears as small ripples at the shoulders of the pass band, thereby increasing insertion loss in the pass band.
The normal-type IDT herein refers to an IDT in which all the electrode fingers defining the IDT have equal overlap lengths.
On the other hand, conventionally, an implementation is known in which the overlap lengths (apertures) of the electrode fingers of the IDT defining a surface acoustic wave filter are varied and weighted in accordance with particular principles. The weighting method is called the apodization method. An IDT weighted as such is also called an apodized electrode. The apodization method is conventionally known as a technique for suppressing higher-mode waves.
Japanese Unexamined Patent Application Publication No. 9-246911 discloses a surface acoustic wave filter having a ladder-type circuit configuration including the apodized electrodes which are described as being effective to suppress higher-mode waves. The conventional device has an arrangement in which a surface acoustic wave resonator having an IDT weighted by the apodization method is used as either a parallel arm resonator or a series arm resonator. Also disclosed is an arrangement in which 50% or more of the electrode fingers of the parallel arm resonator are weighted by the apodization method, and in which 30% to 80% of the electrode fingers of the series arm resonator are weighted by the apodization method.
Furthermore, the prior art proposes an arrangement in which either the parallel arm resonator or the series arm resonator is weighted by the apodization method while the other is defined by a SAW resonator having a normal-type IDT.
The conventional art describes that through weighting in accordance with the above particular ratio by the apodization method, occurrence of higher-mode surface acoustic waves, which cause ripples within the pass band of transmission characteristics, is suppressed, and thereby desirable transmission characteristics are obtained.
The method disclosed in Japanese Unexamined Patent Application Publication No. 9-246911 describes that 30% to 80% of the pairs of the electrode fingers of the IDT electrode of the series arm resonator are to be weighted because 30% or less is not sufficiently effective, while 80% or more causes new ripples in the pass band thereby worsening the loss.
As described above, in the surface acoustic wave filter having the conventional ladder-type circuit configuration, disclosed in Japanese Patent No. 56-19765, the higher-mode waves generate ripples at the shoulders of the pass band, narrowing the pass band and worsening the insertion loss.
On the other hand, the surface acoustic wave filter disclosed in Japanese Unexamined Patent Application Publication No. 9-246911 suppresses higher-mode ripples by the above-described weighting. However, although the higher-mode ripples are suppressed, the minimum insertion loss within the pass band remains almost unchanged, or is even smaller.
Band-pass filters to be used in cellular phones, and other suitable devices absolutely require not only small loss within the pass band but also small maximum variation of loss within the pass band. Therefore, an ideal band-pass filter is a filter which provides uniform loss over the entire pass band.
The surface acoustic wave filter disclosed in Japanese Unexamined Patent Application Publication No. 9-246911 suppresses higher-mode ripples as described above, thereby broadening the pass band and providing improved insertion loss characteristics within the pass band. However, the minimum insertion loss remains unchanged or is even smaller. As a result, the variation of loss within the pass band is not reduced.
To overcome the above-described problems with the prior art, preferred embodiments of the present invention provide a surface acoustic wave filter which eliminates the above-described shortcoming, suppressing ripples caused by higher-mode, broadening the pass band, and reducing the variation of loss within the pass band.
The inventors, through research aimed at eliminating the problems with the prior art, discovered that careful weighting by the apodization method greatly broadens the pass band while minimizing and suppressing ripples caused by higher-mode and also greatly reduces the variation of loss within the pass band, thereby achieving the various advantages of the present invention.
That is, in the arrangement disclosed in Japanese Unexamined Patent Application Publication No. 9-246911, weighting by the apodization method is conducted to maximize suppression of ripples caused by higher-mode waves, which appears on the high-frequency side of the pass band. On the other hand, in accordance with preferred embodiments of the present invention, ripples are intentionally generated within the pass band by the apodization method while optimizing the ripple frequency, and greatly reducing the variation of loss within the pass band.
A surface acoustic wave filter according to at least one preferred embodiment of the present invention includes a piezoelectric substrate and at least two series arm resonators and at least one parallel arm resonator each defined by a one-port surface acoustic wave resonator, disposed on the piezoelectric substrate. The at least two series arm resonators and at least one parallel arm resonator are connected to define a ladder-type circuit. Each of the at least two series arm resonators includes an interdigital transducer weighted by apodization, and the apodizations for the at least two series arm resonators are different from each other.
It is preferable that at least about 80% of electrode fingers of the interdigital transducers of the at least two series arm resonators are apodized.
A ratio between the maximum value and the minimum value of overlapping length in the apodization is preferably different between the at least two series arm resonators. The difference between the two ratios is preferably at least about 0.1, and more preferably at least about 0.2.
Frequency positions of ripples caused by the at least two series arm resonators are preferably different from one another.
The surface acoustic wave filter is preferably used in an antenna duplexer and a communication device.
According to preferred embodiments of the present invention, the frequency positions of the ripples generated by a plurality of serial-arm resonators are varied, such that, with respect to the filter characteristics of the ladder-type surface-acoustic-wave filter, variation of insertion loss within the pass band is greatly reduced which greatly enhances evenness within the pass band.
For the purpose of illustrating the invention, there is shown in the drawings several embodiments which are presently preferred, however, it being understood that the invention is not limited to the precise arrangements and instrumentalities shown.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the detailed description of preferred embodiments below with reference to the attached drawings.